1. Field of the Invention
This invention relates to a hydrophilic porous membrane and a method for the manufacture of the membrane, and more particularly relates to a hydrophilic porous membrane obtained by the impartation of hydrophilicity to the surface of a hydrophobic porous membrane and excelling in perviousness to water and mechanical strength.
2. Description of Prior Art
Heretofore, cellulose derivatives possessing high perviousness to water, particularly porous membranes of cellulose acetate, have prevailed as high molecular porous membranes for use in various applications to filtration and dialysis, for example. Unfortunately, these cellulose derivatives find utility in a greatly limited range of applications because they are deficient in resistance to acids, alkalis, and organic solvents and are easily deformed by heat and pressure.
To take the place of these porous membranes of cellulose derivatives, porous membranes made of non-cellulose type synthetic resins. They are widely varied in type, embracing cross-linked water-soluble high molecular compounds, copolymers of hydrophilic monomers and hydrophobic monomers, homogeneous polymers from polar monomers, hydrophobic polymers caused to incorporate therein a hydrophilic group, and aromatic and heterocyclic condensates.
Among other porous membranes of such non-cellulose type synthetic resins, those of hydrophobic porous membrane incorporating therein a hydrophilic group have the advantage that generally the membranes themselves excel in various properties such as strength and resistance to chemicals. So long as the impartation of hydrophilicity is safely attained as expected, therefore, these hydrophobic polymers are allowed to produce porous membranes which enjoy excellent balance between perviousness to water and imperviousness to substances subjected to removal and possess high strength.
As means of imparting hydrophilicity to hydrophobic porous membranes, the method which effects impartation of a hydrophilic group to the surface of a membrane by teating the membrane with an aqueous solution of such alkali hydroxide as sodium hydroxide or potassium hydroxide (Japanese Patent Laid-Open SHO 58(1983)-93,734) has been known in the art. This method has the problem that the conditions for the effective use of the membrane are not easily managed because the membrane possibly loses strength because of the action of the alkali. The method which effects the impartation of hydrophilicity by graft polymerizing a hydrophilic monomer to the surface of a hydrophobic polymer (Japanese Patent Publication SHO 56(1981)-44,098) has also been known in the art. This method has a fair possibility that since the hydrophobic polymer is a porous substance, the graft polymerization does not easily proceed uniformly even in the pores. The method which effects the impartation of hydrophilicity by immersing a hydrophobic porous membrane in an alcohol, then treating the membrane with an aqueous solution of a water-soluble polymer, drying the treated membrane, and thereafter insolubilizing the water-soluble polymer deposited on the membrane by a treatment with heat or radiation (Japanese Patent Laid-Open SHO 54(1979)-17,978) has been known also in the art. This method has the disadvantage that much time is spent between the time the membrane is immersed in the alcohol and the time the alcohol in the membrane is substituted with the aqueous polymer solution and the heat or radiation used during the treatment for insolubilization has a fair possibility of degrading the strength of membrane and varying the diameter of pores in the membrane. In the existing circumstance, therefore, a porous membrane which enjoys excellent balance between perviousness to water and imperviousness to substances subjected to removal and possesses high strength cannot be obtained by imparting a hydrophilic group to a hydrophobic porous membrane.
An object of this invention, therefore, is to provide a novel hydrophilic porous membrane and a method for the manufacture of the membrane.
Another object of this invention is to provide a hydrophilic porous membrane which is obtained by imparting hydrophilicity uniformly to the surface of a hydrophobic porous membrane and which excels in perviousness of water and mechanical strength and a method for the manufacture of the membrane.
Still another object of this invention is to provide an inexpensive hydrophilic porous membrane which has undergone easy and quick impartation of hydrophilicity at no sacrifice of the strength inherent in the substrate of membrane and a method for the manufacture of the membrane.